1. Field of the Invention
The present invention relates to a process for producing or repairing components of turbine engines, especially gas turbines. The process may be used to manufacture or repair the turbine or compressor or fan blades or vanes as such, or may be used to manufacture a rotor in which the turbine or compressor blades are integrally formed with the rotor disk.
2. Prior Art
Gas turbine engines have three main sections, namely fan, compressor and turbine, each of which may have several stages connected through a central shaft. Each stage has one rotor and one stator. Each rotor used in a gas turbine engine consists of a disk fastened mechanically to a central shaft and blades of airfoil shape attached mechanically to the rim of the disk. Each stator has vanes, also of airfoil shape, attached at an outer end to the engine casing and at the inner end to a shroud. Depending on the size of the engine, each rotor and stator may contain dozens of blades or vanes. The present invention is primarily concerned with manufacture and repair of the rotor blades, especially of the turbine section, which are subject to high heat and stress, but may also be used to manufacture or repair the stator vanes. The term "turbine/compressor blades" as used herein is intended to include the turbine and compressor and also fan rotor blades, and the vanes used in the fan, compressor and turbine stages.
Normally, the blades and disk of each rotor are manufactured separately. Individual blades are made using a number of processes including hot forging, investment casting, directional solidification of melts, etc., depending on the material and functional requirements. For attaching the blades to the disks, either "dove-tail" or "fir-tree" geometry is imparted to the base of the blades, during casting or forging, and may require post machining. The disk is usually forged, and slots of suitable dove-tail or fir-tree shape for the blade attachments are machined. The final operation is the assembly of parts to form a turbine.
The turbine blades may be hollow, with acute angled holes on the leading and trailing edges as well as on the walls and tip. The cooling holes are now often drilled by a high powered Nd:YAG laser. The hollow geometry with cooling holes helps keep the blade material cooler under the operating conditions, and thereby maximises the operating life.
The conventional process for making turbine/compressor rotors has the following drawbacks:
1. The various processes of making the blades, whether by forging, investment casting, or directional solidification, and the subsequent machining, are expensive;
2. Since the blades are attached mechanically to the disks, considerable cost is involved with joint preparation, both for the joint parts of the blades and of the disk. Accurate assembly is required to maintain the desired orientation of the blades. The joints between the blades and the disks are subject to fretting fatigue at the interfaces of the joints and this reduces the life of the rotor.
3. The drilling of cooling holes in the blades is an expensive process and there are problems with drilling acute angled holes required by newer designs of blades. Also, there is a limit to the smallness of hole diameter which can be produced by laser drilling; it would be preferable to use a large number of holes smaller than those which can be drilled by a laser.
Attempts have been made to produce turbine/compressor blades by a process analogous to laser cladding or welding in which a laser is traversed over a metal surface while powdered metal is supplied to the surface so that the added metal is fused to the underlying surface. By this means layers of metal can be built up to form an article having a shape determined by a computer-guided laser and metal delivery means.
Such attempts have been made by Sandia National Laboratories, of Albuquerque, N.Mex., as described in a paper entitled "Laser Engineered Net Shaping (LENS) for Additive Component Processing" by Dave Keicher, presented at a conference entitled "Rapid Prototyping and Manufacturing '96" held by SME at Dearborn, Mich., U.S.A., in April 1996. Initially, experiments were made with a single point, off-axis (side) powder delivery nozzle, but this was found to give strong directional dependence on the deposition height. The single side powder nozzle was abandoned in favour of a co-axial powder feed in which single laser is used normal to the workpiece surface being coated and which is co-axially surrounded by a series of powder delivery tubes all feeding into the region at which the laser beam strikes the workpiece. In a later 1998 paper from the same laboratories it was stated that, with the coaxial powder feed arrangement, the best surface finish achieved was 8 micrometers Ra (roughness average) on the walls; this was after years of development.
Such attempts have been made by Sandia National Laboratories, of Albuquerque, N.Mex., as described in a paper entitled "Laser Engineered Net Shaping (LENS) for Additive Component Processing" by Dave Keicher, presented at a conference entitled "Rapid Prototyping and Manufacturing '96" held by SME at Dearborn, Mich., U.S.A., in April 1996. Initially, experiments were made with a single point, off-axis (side) powder delivery nozzle, but this was found to give strong directional dependence on the deposition height. The single side powder nozzle was abandoned in favour of a co-axial powder feed in which a single laser is used normal to the workpiece surface being coated and which is co-axially surrounded by a series of powder delivery tubes all feeding into the region at which the laser beam strikes the workpiece. In a later 1998 paper from the same laboratories it was stated that, with the coaxial powder feed arrangement, the best surface finish achieved was 8 micrometers Ra (roughness average) on the walls; this was after years of development.
Other processes for producing turbine blades by laser welding or deposition are described in the following patent publications:
U.S. Pat. No. 5,160,822, which issued Nov. 3, 1992 to Aleshin;
U.S. Pat. No. 5,900,170, which issued May 4, 1999 to Marcin, Jr., et al.;
Can. Pat. Appln. No. 2,012,449 to Rathi et al., published Nov. 15, 1990;
Can. Pat. Appln. No. 2,085,826 to Williams, published Jun. 20, 1993; and
Can. Pat. Appln. No. 2,170,875 to Goodwater et al., published Mar. 9, 1995.
In addition, U.S. Pat. No. 5,038,014, issued Aug. 4, 1991 to Pratt et al., describes a laser welding technique for making turbine or compressor blades, which is said to be suitable also for forming the rotor blades integrally with the rotor disk. The patent suggests using a conventional laser cladding process with a normal or vertical laser beam and a powder feed tube set at an angle. It is evident from tests done by applicants that there are major problems with this method:
1) The height of the airfoil will be uneven due to the multi-directional nature of the beads used to build the blade and the fact that this gives uneven deposition, and
2) The surface finish will be very poor, and it is expected that machining will be necessary.
The present invention provides a process which can be used either to produce or to repair blades of rotors or vanes of stators used in gas turbines and other turbines by addition of metal to a base using a laser process similar to those discussed above, but having different laser/metal delivery configurations. The process can produce parts with such accuracy that machining may be avoided. The basic process will be referred to herein as "laser consolidation". However, it will be noted that in the literature and patents referred to the same basic process of building a component has been referred to by many different names, e.g. "laser engineered net shaping", "directed light fabrication", "linear translational laser welding", "energy beam deposition", "sequential layer deposition", "energy beam casting", and "laser sintering". The term "laser consolidation" is intended to include processes of this type in which a laser beam is used to melt metal, delivered in powder or wire form, to a surface, to build up a shaped object by controlling movement of the beam and metal delivery means.